The OP here. Great suggestions. However, my concern is not simply how
to make the cat hear me. It is doing so without making a nuisance of
myself all over the neighborhood. Maybe I am self-conscious, but I
would like to be as discreet towards other humans as possible.

I am taking care of a neighbor's cat for about 6 months while they are
out of the country. She spends the days outside. I would like to ring
the whistle in my backyard and have her come, without bothering the
neighbors everyday. Even more so if I have to walk the block blowing my
whistle.